This question asks about using an ice cream scoop on Shabbath, and an internet forum linked in the question has a discussion about the permissibility of decorative plating of food generally. According to one commenter, one is not even allowed to place cookies on a tray in an attractive manner. What is the source for this prohibition (and is it a correct interpretation of the source)?

However, the comment does not represent a correct interpretation of this source, which is specifically referring to molding a mass of food to make it look nice. This halacha is based on the Magein Avraham (340:17), who writes that mushing pieces of food together into a decorative shape presents a problem of boneh.

Incidentally, some other types of decorative food preparation might pose problems of m'marei'ach d'rabbanan (see for e.g. Bi'ur Halacha at the end of 321, s.v. b'ma'achal) or koseiv d'rabanan (see for e.g. Sha'ar HaTziyun 500:20). However, simply arranging individual cookies in an attractive manner on a tray poses no problem on Shabbos or Yom Tov (so far as I know).